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Navigation

NAU 102

Lesson 15

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Magnetic Compass

Basic Principle

Small magnets are placed under a

compass card.

The card is free to move and

aligns with the horizontalcomponent of the Earth’s

magnetic field.

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Magnetic Compass

Early Compasses

A lodestone placed on a chip of

wood, floating in a bowl of water.

Modern Compasses

Basically the same. Just more

parts!

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Marine Compasses

Many ships will have two compasses:

Standard Compass

Located on the centerline, away from

magnetic influences.

Directions stated as “per standardcompass” (psc)

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Marine Compasses

Many ships will have two compasses:

Steering Compass

Located near the helm console.

Directions stated as “per steeringcompass” (p stg c)

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Compass Components

Bowl

Contains and supports all other

compass components

Constructed of non-magnetic

materialGlass cover is secured with a Bezel

Ring.

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Bowl & Bezel Ring

Bowl

Bezel

Ring

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Compass Components

Lubber’s Line

Aligned with the centerline of

the vessel.

Indicates the compass heading ofthe vessel.

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Lubber’s Line

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Compass Components

Compass Card

Stays aligned with magnetic

meridians.

Usually graduated in 360° andcardinal and intercardinal points.

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Compass Card

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Compass Components

Directive Magnets

Two or more magnets, mountedunderneath the compass card.

Align the card with the magneticmeridian.

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Directive Magnets

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Compass Components

Pivot

Pin in bowl which supports the

compass card.

Compass card rotates on a jeweled bearing.

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Pivot

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Compass Components

Compass Fluid

Clear liquid, resistant to freezing.

Usually glycol or petroleum based.

Reduces friction and oscillations.

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Compass Components

Card Float or Air Chamber

Attached to the card.

Supports most of the weight of the

card.

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Compass Components

Expansion Bellows

Allow the fluid to expand and

contract with temperature.

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Pivot

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Mounting Components

The compass is mounted on gimbals.

Double, hinged rings of non-

magnetic material.

Allow the compass to remain level as

the ship rolls and pitches.

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Gimbals

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Mounting Components

Gimbaled compass is

mounted in a Binnacle.

Supports the compass.

Protects fromenvironment and

magnetic interference

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Binnacle Components

Cover (Hood)

Protects the compass from the

environment.

Spider Glass

Magnifying glass to enable reading of

the compass.

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Binnacle Components

Light

Enables the use of the compass at

night.

Correctors

Minimizes the effects of magnetic

interference (deviation).

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Correctors

Quadrantal Spheres

Soft Iron Spheres

Reduce deviation on

intercardinal

headings

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Correctors

Fore and Aft /

Athwartships Magnets

Reduce deviation on

cardinal headings

Magnets placed in

movable trays under

the compass

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Correctors

Heeling Magnet

Vertically

suspended

beneath the

compass.

Corrects compass errors caused by

rolling and pitching.

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Correctors

Compensates for induced

magnetism caused by verticalcomponent of force.

Flinders Bar

Soft iron contained in a tube on the

outside of the binnacle.

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Binnacle

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Introduction to Navigation

Questions?